Wide area electron beam generating assemblies are known, for example, for generating X-rays to preionize a gas laser. One form of known assembly is a corona-plasma cathode which employs an insulating dielectric tube which contains an earthed metal liner. A metal wire array is wrapped around the tube, an earthed anode is placed close to but spaced from the wire array and the whole is housed in a container under vacuum. In operation of this known assembly a pulsed high negative voltage is applied to the wire array and gas ionization takes place at the wire-dielectric-vacuum interface to form a plasma. This interface is capacitatively ballasted to the metal liner and the plasma spreads out on the dielectric tube surface. An electron beam is driven out of the plasma to the anode under the influence of the applied field. The accelerating voltage across the anode-cathode gap is the same as that across the dielectric tube.
Such a known form of assembly has several disadvantages. A first disadvantage is that the wire array wound around the dielectric tube is prone to breakage due to erosion, causing the broken ends to spring away from the dielectric tube and the remaining wire turns to unwind from the tube with consequent failure of the assembly. Such breakage of the wire array can happen after only short periods of operation leading to only a short working life for the assembly. The second disadvantage is that the dielectric tube itself can fail relatively quickly under continuous high repetition rate operation, which is necessary for electron beam energies in excess of 70 keV. Such dielectric tube failure results from stress generated therein by the continuous high repetition rate operation and this in turn results in an undesirable reduction in working life for the assembly.